Method and apparatus for AD conversion, semiconductor device for detecting distribution of physical quantity, and electronic apparatus

ABSTRACT

A column analog-to-digital converter having a voltage comparator and a counter is arranged for each a vertical signal line. The voltage comparator compares a pixel signal inputted via the vertical signal line at each row control signal line with a reference voltage, thereby generating a pulse signal having a length in time axis corresponding to the magnitude of a reset component and a signal component. The counter counts a clock to measure the width of the pulse signal until the end of the comparison operation of the comparator, and stores a count at the end of the comparison. A communication and timing controller controls the voltage comparator and the counter so that, in a first process, the voltage comparator performs a comparison operation on a reset component with the counter performing a down-counting operation, and so that, in a second process, the voltage controller performs the comparison operation on a signal component with the counter performing an up-counting operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an analog-to-digital (A/D) conversionmethod, an A/D converter, a semiconductor device, having a plurality ofunit elements, for detecting a distribution of a physical quantity, andan electronic apparatus. In particular, the present invention relates toa technique for converting an analog output into digital data in asemiconductor device and an electronic apparatus, in which a pluralityof unit elements responsive to an electromagnetic wave inputted from theoutside, such as light or radiation rays, converts the inputelectromagnetic wave into the distribution of the physical quantity asan electrical signal, and the electrical signal is selectively readthrough address control.

2. Description of the Related Art

Semiconductor devices for detecting a distribution of physicalquantities are widely used. Such semiconductor device includes unitelements (such as pixels) responsive to an electromagnetic wave, such aslight or radiation rays, arranged in a line or a matrix.

For example, a charge-coupled device (CCD), a metal-oxide semiconductor(MOS) device, and a complementary metal-oxide semiconductor (CMOS)device, each device for detecting light (one type of the electromagneticwaves) as a physical quantity, are typically used in the video equipmentfield. After being converted into an electrical signal by a unit element(pixel in the solid-state image pickup device), these devices read thedistribution of the physical quantity.

Some solid-state image pickup devices have pixels, each pixel includingamplifying solid-state image pickup element (active pixel sensor or alsoreferred to as gain cell). The active pixel sensor includes a drivetransistor for generating a pixel signal in response to a signal chargegenerated in a charge generator. Many of the CMOS solid-state imagepickup devices have such a structure.

To read the pixel signal in the amplifying solid-state image pickupdevice, address control is performed on a pixel area having a pluralityof pixels so that a signal is read from any individual pixel. Theamplifying solid-state image pickup device is thus an address controltype solid-state image pickup device.

The amplifying solid-state image pickup device, which is one type of X-Yaddressing solid-state image pickup devices having a matrix of unitpixels, is MOS structured with a MOS transistor in each pixel to impartamplification capability to the pixel. More specifically, an activeelement amplifies a signal charge (photoelectron) stored in a photodiodeas a photoelectric conversion element, and the amplified signal is read.

In such an X-Y addressing solid-state image pickup device, a largenumber of pixel transistors are arranged in a two-dimensional matrix toform a pixel area. The storage of signal charge starts in the pixels ona per line (row) basis or individually. A current signal or a voltagesignal responsive to the stored signal charge is successively read byaddressing. In the MOS types (including the CMOS type), in one of thetypical address controls, one row of pixels is concurrently accessed anda pixel signal on a per row basis is read from the pixel area.

An analog pixel signal read from the pixel area is converted intodigital data by an analog-to-digital (A/D) converter as necessary. Sincethe output pixel signal is a mixture of a signal component and a resetcomponent, an effective signal component needs to be extracted by takinga difference between a signal voltage corresponding to the resetcomponent and a signal voltage corresponding to the signal component.

The same is true when the analog pixel signal is converted to thedigital signal. The difference component between the signal voltagecorresponding to the resent component and the signal voltagecorresponding to the signal component need to be converted into digitaldata. To this end, a variety of A/D conversion mechanisms have beenproposed in the following patent documents and non-patent documents:

Patent document 1: Japanese Unexamined Patent Application PublicationNo. 11-331883;

Non-patent document 1: W. Yang et. al., “An Integrated 800×600 CMOSImage System,” ISSCC Digest of Technical Papers, pp. 304-305, February,1999;

Non-patent document 2: Kazuya YONEMOTO, “CCD/CMO Image Sensor no kiso toouyo”, CQ Publishing Co., Ltd, first edition p201-203, Aug. 10, 2003;

Non-patent document 3: Toshifumi IMAMURA, and Yoshiko YAMAMOTO, “3.Kosoku kinou CMOS image sensor no kenkyu”, Internet

<URL:http://www.sankaken.gr.jp/project/iwataPJ/report/h12/h12index.html>

on-line searched Mar. 15, 2004;

Non-patent document 4: Toshifumi IMAMURA, Yoshiko YAMAMOTO, and NaoyaHASEGAWA “3. Koshoku kinou CMOS image sensor no kenkyu”, Internet

<URL:http://www.sankaken.gr.jp/project/iwataPj/report/h12/h14index.html>on-line searched Mar. 15, 2004; and

Non-patent document 5: Oh-Bong Kwon et. al., “A Novel Double SlopeAnalog-to-Digital Converter for a High-Quality 640×480 CMOS ImagingSystem,” VL3-03 1999 IEEE p. 335-338.

The A/D conversion mechanisms disclosed in these documents havedifficulty in terms of the size and area of a circuit, powerconsumption, the number of interfacing lines with another functionalblock, noise picked up by the lines, and current consumption.

FIG. 6 illustrates a known CMOS solid-state image pickup device 1 (CMOSimage sensor) having an A/D converter and a pixel area mounted on thesame semiconductor substrate. As shown in FIG. 6, the solid-state imagepickup device 1 includes a pixel array (image pickup area) 10 having aplurality of unit pixels 3 arranged in a matrix, a driver 7 arrangedoutside the pixel array 10, a counter (CNT) 24, a column processor 26including a column A/D converter 25 arranged on a per column basis, areference signal generator 27 including a DAC (digital-to-analogconverter) for supplying a reference voltage for D/A conversion to thecolumn A/D converter 25 in the column processor 26, and an outputcircuit 28 including a subtracter 29.

The driver 7 includes a horizontal (row) scanning circuit 12 forcontrolling column addresses and column scanning, a vertical (row)scanning circuit 14 for controlling row addresses and row scanning, anda timing controller 21 for receiving a master clock CLK0 via a terminal5 a, and for generating a variety of internal clocks to control thehorizontal scanning circuit 12 and the vertical scanning circuit 14.

Each unit pixel 3 is connected to a row control line 15 controlled bythe vertical scanning circuit 14 and a vertical signal line 19 fortransferring a pixel signal to the column processor 26.

The column A/D converter 25 includes a voltage comparator 252 and a datamemory 255. The voltage comparator 252 compares a reference voltage RAMPgenerated by the reference signal generator 27 with an analog pixelsignal obtained via the vertical signal line 19 (V0, V1, . . . ) fromthe unit pixel 3 on a per row control line 15 (H0, H1, . . . ). The datamemory 255 includes latches (flipflops) as a memory for storing a countof the counter 24 that counts time until the voltage comparator 252completes the comparison process. The column A/D converter 25 has afunction of n bit A/D conversion. The data memory 255 includes a latch 1and a latch 2, each having n bit capacity, as an independent internalmemory area.

The ramp reference voltage RAMP generated by the reference signalgenerator 27 is commonly fed to input RAMP terminals of the voltagecomparators 252 while respective pixel signal voltages from the pixelarray 10 are supplied to the other terminals of the voltage comparators252 connected to the vertical signal lines 19 of the correspondingcolumn lines. The output of the voltage comparator 252 is supplied tothe data memory 255.

The counter 24 performs a count process in accordance with a count clockCLK0 corresponding to the master clock CK0 (both clocks having the sameclock frequency). The counter 24 thus outputs count output CK1, CK2, . .. , CKn together with the count clock CK0 to each column A/D converter25 in the column processor 26.

The lines for the count outputs CK1, CK2, . . . , CKn from the counter24 are routed to the latches of the data memory 255 arranged for eachcolumn. The column A/D converter 25 thus operates commonly in responseto the single counter 24.

The output of the column A/D converter 25 is connected to a horizontalsignal line 18. The horizontal signal line 18 include signal linescovering 2n bit width, and is connected to the subtracter 29 in theoutput circuit 28 via 2n sense circuits (not shown) corresponding to theoutput lines of the horizontal signals of the horizontal signal line 18.Video data D1 output from the output circuit 28 is output to the outsidevia an output terminal 5 c from the solid-state image pickup device 1.

FIG. 7 is a timing diagram illustrating operation of the solid-stateimage pickup device 1 of FIG. 6.

The count of the counter 24 is reset to an initial value “0” for a firstreading operation. After the first reading from the unit pixel 3 of anyrow Hx to the vertical signal line 19 (V0, V1, . . . ) is stabilized,the reference voltage RAMP that is ramped in time axis by the referencesignal generator 27 is input. The voltage comparator 252 compares thereference voltage RAMP with a pixel signal voltage of any verticalsignal line 19 (column number Vx).

The counter 24 starts counting a conversion time of the voltagecomparator 252 at the moment the reference voltage RAMP is inputted tothe one terminal RAMP of each of the voltage comparator 252. Insynchronization with the ramp waveform voltage issued from the referencesignal generator 27 (time t10), the counter 24 starts down-counting fromthe initial value “0” in a first counting operation.

The voltage comparator 252 compares the ramp reference voltage RAMP fromthe reference signal generator 27 with a pixel signal voltage Vx inputvia the vertical signal line 19. When both voltages become equal to eachother, the voltage comparator 252 inverts the comparator output from ahigh level to a low level (at time t12).

At about the same time as the inversion of the comparator output, thedata memory 255 latches in the latch 1 thereof the count outputs CK1,CK2, . . . , CKn from the counter 24 in response to the comparison timein synchronization with the count clock CK0. A first conversionoperation is thus complete (at time t12).

When a predetermined down-count period has elapsed (at time t14), thetiming controller 21 stops supplying control data to the voltagecomparator 252 and supplying the count clock CK0 to the counter 24. Thevoltage comparator 252 thus stops the comparison operation thereof.

During the first reading operation, a reset component ΔV of the unitpixel 3 is read. The reset component ΔV contains noise varying from unitpixel 3 to unit pixel 3 as an offset. The variations of the resetcomponent ΔV is typically small, and a reset level is common to allpixels. The output of any vertical signal line 19 (Vx) is largely known.

The comparison period during the first reading of the reset component ΔVis shortened by adjusting the reference voltage RAMP. In this knownexample, the reset component ΔV is adjusted within a count period of 7bits (128 clocks).

In the second reading operation, a signal component Vsig responsive toan amount of incident light per unit pixel 3 is read in addition to thereset component ΔV, and the same operation as in the first readingoperation is performed.

The count of the counter 24 is reset to “0” for a second readingoperation. After the second reading from the unit pixel 3 of any row Hxto the vertical signal line 19 (V0, V1, . . . ) is stabilized, thereference voltage RAMP that is ramped in time axis by the referencesignal generator 27 is input. The voltage comparator 252 compares thereference voltage RAMP with a pixel signal voltage of any verticalsignal line 19 (column number Vx).

The counter 24 starts counting a conversion time of the voltagecomparator 252 at the moment the reference voltage RAMP is inputted tothe one terminal RAMP of each of the voltage comparator 252. Insynchronization with the ramp waveform voltage issued from the referencesignal generator 27 (time t20), the counter 24 starts down-counting fromthe initial value “0” in a second counting operation.

The voltage comparator 252 compares the ramp reference voltage RAMP fromthe reference signal generator 27 with a pixel signal voltage Vxinputted via the vertical signal line 19. When both voltages becomeequal to each other, the voltage comparator 252 inverts the comparatoroutput from a high level to a low level (at time t22).

At about the same time as the inversion of the comparator output, thedata memory 255 latches the count outputs CK1, CK2, . . . , CKn from thecounter 24 in response to the comparison time in synchronization withthe count clock CK0. A second conversion operation is thus complete (attime t22).

The data memory 255 latches the first count and the second count in adifferent location thereof, i.e., the latch 2. During the second readingoperation, the reset component ΔV and the signal component Vsig of theunit pixel 3 are read.

When a predetermined down-count period has elapsed (time t24), thetiming controller 21 stops supplying control data to the voltagecomparator 252 and stops supplying the count clock CK0 to the counter24. In this way, the voltage comparator 252 stops the comparisonoperation thereof.

At a predetermined timing (time t28) of the end of the second countingoperation, the timing controller 21 instructs the horizontal scanningcircuit 12 to read pixel data. In response, the horizontal scanningcircuit 12 successively shifts a horizontal selection signal CH(i) to besupplied to the data memory 255 via a control line 12C.

The count stored in the data memory 255, namely, the digital pixel dataof n bits for the first and second reading operations, is successivelyoutput to the subtracter 29 in the output circuit 28 outside the columnprocessor 26.

The n-bit subtracter 29 subtracts the first pixel data representing thereset component ΔV of the unit pixel 3 from the second pixel data as asum of the reset component ΔV and the signal component Vsig of the unitpixel 3 at each pixel position, thereby determining the signal componentVsig of the unit pixel 3.

The same operation is repeated. The output circuit 28 results in a videosignal representing a two-dimensional image.

In the above-described known technique, the counter 24 is commonly usedby the column A/D converters 25 of all columns. Each memory of the datamemory 255 needs to store the first count and the second count. The datamemory 255 uses a pair of n-bit latches for n bit signals (namely, 2nlatches for each bit), thereby requiring a large circuit area. Thisproblem is hereinafter referred to as a first problem.

Not only the line for the synchronization count clock CK0 but also thelines for the count outputs CK1, CK2, . . . , CKn from the counter 24need to be routed to the latches 1 and 2 of the data memory 255. Becauseof this routing, an increase in noise and an increase in powerconsumption become a concern. This problem is hereinafter referred to asa second problem.

Since the first count and the second count are stored in the differentlocations in the data memory 255, 2n signal lines are required totransfer the count results to the data memory 255, and currents involvedincrease accordingly. This problem is hereinafter referred to as a thirdproblem.

The output circuit 28 subtracts the first count from the second countprior to the signal output, and 2n signal lines for transferring thecount to the n bit subtracter 29 in the output circuit 28 at each timeare required. An increase in noise and an increase in power consumptiondue to data transfer become a concern. This problem is hereinafterreferred to as a fourth problem.

A memory for storing the first read results and a memory storing thesecond read results need to be separately arranged in addition to thecounter, in other words, two memories are required. Signal lines fortransferring the n-bit count from the counter to the memories arerequired. Further, 2n bit signal lines for transferring the first countand the second count, each count being n bits, to the subtracter arealso required. The circuit scale and the circuit area are increased,leading to increases in noise, current consumption and powerconsumption.

In a pipeline arrangement that permits the A/D conversion and thereading process to be performed in parallel, a memory for storing A/Dconverted data is required in addition to a memory for storing the countresults. As in the first problem, two memory systems are required,leading to an increase in the circuit area. This problem is hereinafterreferred to a fifth problem.

To overcome the first problem, the previously quoted non-patent document2 has proposed a column A/D converter. The column A/D converter performsa correlated double sampling (CDS) function and an A/D conversionfunction by cascading a counter commonly used for vertical columns witha CDS processing unit and a latch for latching the count of the counterarranged for each column.

To overcome the second problem, the previously quoted patent document 1and non-patent documents 3-5 have proposed an arrangement in which acounter is arranged on a per column basis in the column processor 26 toperform the A/D function.

An A/D converter disclosed in the non-patent document 2 includes acounter for performing a parallel process on the vertical signal lines(for the vertical columns) and an A/D circuit using a latch, andcalculates a difference between a reset component and a signalcomponent. The A/D converter thus converts the input signal into digitaldata while suppressing fixed pattern noise of the pixels. With thisarrangement, no subtraction process is required, and a single countprocess is sufficient. The memory for storing A/D converted data isconstructed of a latch, thereby avoiding an increase in circuit area. Inother words, the first, third, fourth, and fifth problems are overcome.

Since the counter is commonly shared by all vertical column lines in thesame way as show in FIG. 6, n lines for the count outputs CK1, CK2, . .. , CKn from the counter are needed. The second problem is not solved.

In accordance with the technique disclosed by the non-patent documents 3and 4, currents from a plurality of pixels for detecting light areoutputted to an output bus, and summing and subtraction operations areperformed on the currents on the bus. The resulting signal is convertedin a pulse-width signal having a length in time direction. The number ofclocks responsive to the pulse width of the pulse-width signal arecounted in counters arranged in parallel to perform A/D conversion. Thisarrangement eliminates the need for the wiring for the counter output.In other words, the second problem is solved.

However, there is no mention about the handling of a reset component anda signal component, and it is not certain that the technique disclosedin the non-patent documents 3 and 4 solves the first, third, fourth, andfifth problems. Likewise, the non-patent documents 1 and 5 also fail tostate how a reset component and the signal component are handled.

In contrast, the patent document 1 discloses the manner of handling of areset component and a signal component. Using a correlated doublesampling unit, a subtraction process for subtracting digital data of thesignal component from digital data of the reset component is performedevery vertical column. Voltage data of pure image is thus extracted fromthe reset component and the signal component. The fourth problem is thussolved.

With the technique disclosed in the patent document 1, a count signal isgenerated in a count process by an external system interface. One pairof buffers arranged for each vertical column stores a count at themoment the voltage of one of a reset component and a signal componentmatches a reference voltage in a comparison process. The arrangement ofthe A/D conversion is identical to the technique disclosed in thenon-patent document 1 in that a single counter is commonly shared by allvertical column lines. The first, second, third, and fifth problemsremain unsolved.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amechanism to solve at least one of the previously described problems.

The present invention in a first aspect relates to a method foranalog-to-digital converting, into digital data, a difference signalcomponent, between a standard component and a signal component, of ananalog signal to be processed containing the standard component and thesignal component. A signal corresponding to one of the standardcomponent and the signal component is compared with a reference signalfor generating the digital data. A counting operation is performed inone of a down-count mode and an up-count mode in parallel with thecomparison operation, and then the resulting count at the moment thecomparison operation is completed is stored. The count mode is switcheddepending on whether the comparison operation is performed on thestandard component or the signal component.

In a first step, the reference signal for generating the digital data iscompared with one of the standard component and the signal component,different in physical property, of a single signal to be outputted fromthe same unit element such as a pixel and to be processed. The countingoperation is performed in one of the down-count mode and the up-countmode in parallel with the comparison operation. The count at the momentthe comparison operation is completed is stored.

In a second step, the reference signal is compared with the other of thestandard component and the signal component. The counting operation isperformed in the other of the down-count mode and the up-count mode. Thecount at the moment the comparison operation is completed is stored. Thecount stored subsequent to the second step is a difference from thefirst count. By performing the second counting operation with the countmode switched, the second count is obtained as a digital valuecorresponding to the difference between the standard component and thesignal component.

The signal component to be processed in the second step indicates a truesignal component in the signal to be processed, and does not necessarilymean the true signal component itself. In practice, a signal to beprocessed containing a noise component or a reset component is perfectlyacceptable.

The standard component and the signal component are relative values, anda difference component between the standard component and the signalcomponent is a difference between two signal components different inphysical property of a single signal outputted from the same unitelement such as the same pixel.

In the comparison operation of the standard component and the signalcomponent, each of signals corresponding to the standard component andthe signal component is compared with the reference signal that variesin a predetermined slope to detect a point where the signalscorresponding to the standard component and the signal component matchesthe reference signal. The predetermined slope is not limited to aconstant slope. The slope may vary from one value to next in a stepwisemanner among a plurality of values so that the larger the signalcomponent the larger the slope. In this arrangement, dynamic range isincreased.

The counting operation is performed by counting a count clock from thepoint of generation of the reference signal used in the comparisonoperation to the point where the signal corresponding to one of thestandard component and the signal component. The count responsive to themagnitude of one of the standard component and the signal component.

A common up-down counter may be used to perform the counting operationby switching between the down-count mode and the up-count mode. Thecounter circuit is thus made compact. With the two count modesswitchably used, a subtraction process between the standard componentand the signal component is directly performed. No dedicated subtractoris required to determine the difference between the standard componentand the signal component.

The counting operation in the second step is preferably started with thecount stored in the first step. The count stored subsequent to thesecond step becomes a digital value equal to a difference between thesignal component and the standard component.

The comparison operation and the down-count operation are performed onthe standard component in the first step and the comparison operationand the up-count operation are performed on the signal component in thesecond step. In this way, the count held subsequent to the second stepbecomes a digital value obtained as a result of subtracting the standardcomponent from the signal component.

If the signal component appears subsequent to the standard component intime axis in the signal derived from the unit element such as the pixel,the second step is performed on the signal composed of the standardcomponent and the signal component. The count held subsequent to thesecond step represents the signal component of the unit element.

Preferably, one of the first step and the second step is performed onthe standard component in the down-count mode, and the other of thefirst step and the second step is performed on the signal component inthe up-count mode. The count held after the two steps is a positivedigital value as a result of subtracting the standard component from thesignal component.

The comparison operation and the down-count operation are performed onthe standard component in the first step and the comparison operationand the up-count operation are performed on the signal component in thesecond step. In this way, the count held subsequent to the second stepbecomes a digital value having a positive value obtained as a result ofsubtracting the standard component from the signal component. If thesignal of the unit element contains the standard component and thesignal component appearing subsequent to the standard component in timeaxis, digital data representing an effective signal component of theunit element is obtained as a positive value.

In the comparison of the standard component with the signal component,the standard component is largely constant and the amount of thestandard component is small. The signal component is a variablecomponent obtained when an electromagnetic wave such as light isdetected by the unit element such as a pixel. The maximum amount of thesignal component becomes large. Preferably, the maximum period of thecomparison operation for the standard component is set to be shorterthan the maximum period of the comparison operation of the signalcomponent so that the total of the two A/D conversion periods isshortened.

Preferably, the count stored in the second step in connection with onesignal processed in a preceding cycle of the first step and the secondstep is stored in another data memory, and when the first step and thesecond step are performed on the other signal to be processed a currentcycle, the count is read from the other data memory in parallel.

Preferably, the signal to be processed contains an analog unit signalgenerated and output in the direction of columns by a unit signalgenerator in a semiconductor device for detecting a distribution of aphysical quantity, wherein the semiconductor device includes a matrix ofunit elements, each unit element including a charge generator forgenerating a charge corresponding to an input electromagnetic wave, andthe unit signal generator for generating the analog unit signalcorresponding to the charge generated by the charge generator.

When the unit elements are arranged in a two-dimensional matrix, theanalog unit signal generated and outputted in the direction of columnsby the unit signal generator is accessed and scanned on a per row basis(on column parallel). The first step and the second step are performedon each unit element on a per row basis, thereby speeding up the readingprocess and the A/D conversion process of the unit signal.

The present invention in a second aspect relates to a converter foranalog-to-digital converting, into digital data, a difference signalcomponent, between a standard component and a signal component, of ananalog signal to be processed containing the standard component and thesignal component. The converter includes a comparator for comparing asignal corresponding to each of the standard component and the signalcomponent with a reference signal for generating the digital data, and acounter for performing a counting operation in one of a down-count modeand an up-count mode in parallel with the signal comparison operationperformed by the comparator and for storing the resulting count at themoment the comparison operation is completed.

Preferably, the converter includes a controller for switching the countmode of the counter depending on whether the comparator performs thecomparison operation on the standard component or the signal component.

The counter may include a counting circuit that switchably performs thecounting operation between an up-count mode and a down-count mode. Thecounter may include a down-counter that performs the counting operationin the down-count mode and an up-counter that performs the countingoperation in the up-count mode. In the latter case, the counter mayfurther include an adder circuit that sums the count of the down-counterand the count of the up-counter.

Each of a semiconductor and an electronic apparatus of the presentinvention performs the above-referenced A/D conversion method, and isidentical in structure to the above-referenced A/D converter.

In accordance with the semiconductor device of the present invention, aplurality of A/D converters, each including a comparator and a counter,are arranged in the direction of columns and rows of the unit elements.

Preferably, the comparator captures the analog unit signal generated andoutput in the direction of columns by the unit signal generator on a perrow basis, and the comparator and the counter perform respectiveprocesses for each unit element on a per row basis. Preferably, the unitsignal generator includes a semiconductor element for amplification.

Preferably, the charge generator includes a photoelectric conversionelement that receives light as an electromagnetic wave and generates acharge responsive to the received light. With this arrangement, thesemiconductor device is arranged as a solid-state image pickup device.

In accordance with the present invention, the reference signal for theA/D conversion is compared with the signal to be processed containingthe standard component and the signal component, and the countingoperation is performed in one of the down-count mode and the up-countmode in parallel with the comparison operation. When the count at theend of the comparison operation is stored, the count mode is switcheddepending on whether the comparison operation is performed on thestandard component or the signal component.

In this way, the digital data representing the difference between thestandard component and the signal component is obtained as a result ofthe counting operation of the two modes, namely, the down-count mode andthe up-count mode.

The function of the memory storing the counts of the standard componentand the signal component is performed by a latch owned by the counter.This arrangement eliminates the need for arranging a dedicated memoryfor storing the A/D converted data separate from the counter. Theproblem of increases in circuit size and circuit area is thus solved.

Since the A/D converter is composed of the comparator and the counterregardless of whether the common up-down counter is used, the countingoperation is controlled by a single count clock line for operating thecounter regardless of the number of bits and a control line forswitching the count mode. Signal lines for transferring the count of thecounter to the memory are thus eliminated. This arrangement overcome theproblem of an increase in noise level and an increase in powerconsumption.

The up-down counter is commonly used with the count mode switched. Thesubtraction operation is directly performed by the counter. No dedicatedsubtracter is required for determining the difference between thestandard component and the signal component. Also, this arrangementwithout the need for transferring the data to a subtracter avoidsincreases in noise level, current, and power consumption.

If the counter is composed of a down-counter and an up-counter, thecount reached at the first step is set prior to the second countingoperation. The subtraction operation between the standard component andthe signal component is directly performed. No dedicated subtracter isrequired for determining the difference between the standard componentand the signal component. Also, this arrangement without the need fortransferring the data to a subtracter avoids increases in noise level,current, and power consumption.

When the counter is composed of a down-counter and an up-counter, thecounting operation may be started with zero rather with the countreached at the first step. In such a case, an adder is required. Sincethe adder is arranged in each A/D converter containing the comparatorand the counter, a length of line to the adder is shortened. Increasesin noise level, current, and power consumption are still controlled.

The A/D converter is composed of the comparator and the counter in pair.When the unit signal outputted by the semiconductor device having anarray of unit elements is to be processed, each of a plurality of A/Dconverters arranged in the direction of rows (the direction of an arrayof unit element columns) are provided with a counter. This arrangementeliminates the need for routing the output lines of the counter to thelatch as shown in FIG. 6, thereby avoiding the problem of increases innoise level and power consumption.

In a pipeline operation in which the A/D conversion process and thereading process are performed in parallel, a single memory for storingthe A/D converted data for each A/D converter is sufficient. An increasein the circuit area is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a circuit of a CMOS solid-state imagepickup device in accordance with a first embodiment of the presentinvention;

FIG. 2 is a timing diagram illustrating operation of a column A/Dconverter in the solid-state image pickup device of FIG. 1 in accordancewith the first embodiment of the present invention;

FIG. 3 is a block diagram of a CMOS solid-state image pickup device inaccordance with a second embodiment of the present invention;

FIG. 4 is a timing diagram illustrating operation of the column A/Dconverter in the solid-state image pickup device in accordance with thesecond embodiment of the present invention;

FIGS. 5A-5C are block diagrams illustrating counters;

FIG. 6 is a block diagram of a known CMOS solid-state image pickupdevice having an A/D converter and a pixel section arranged on the samesemiconductor substrate; and

FIG. 7 is a timing diagram illustrating operation of the knownsolid-state image pickup device of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of a CMOS solid-state image pickup device in accordancewith the present invention are described below with reference to thedrawings. The CMOS solid-state image pickup device is one example of X-Yaddressing solid-state image pickup devices. All pixels of the CMOSsolid-state image pickup device are constructed of NMOS.

The present invention is not limited to the image pickup device of MOStype. All embodiments described later are applicable to anysemiconductor device for detecting physical property, including a lineor a matrix of unit elements responsive to an electromagnetic wave, suchas light or a radiation ray, inputted from the outside.

FIG. 1 illustrates a CMOS solid-state image pickup device (CMOS imagesensor) 1 in accordance with a first embodiment of the presentinvention. The CMOS solid-state image pickup device 1 is one example ofan electronic apparatus of the present invention.

The solid-state image pickup device 1 includes a pixel section having atwo-dimensional matrix of pixels, each pixel forming a light receivingelement (as one example of a charge generator) that outputs a voltagesignal responsive to an input amount of light, and correlated doublesampling (CDS) units and analog-to-digital (A/D) converters arranged ina column parallel fashion.

The “arrangement in the column parallel fashion” means that a pluralityof CDS units and a plurality of A/D converters are arranged in parallelwith respective vertical signal lines 19. The plurality of theseelements may be arranged on one edge of the columns of pixels in thepixel array 10 (i.e., on the output side at the lower side of the pixelarray 10 as shown in FIG. 1) or may be arranged on both one edge of thecolumns of pixels in the pixel array 10 (i.e., on the output side at thelower side of the pixel array 10 as shown in FIG. 1) and the other edgeof the columns of the pixels in the pixel array 10 (i.e., on the upperside of the pixel array 10). In the latter case, horizontal scanningunits for scanning the pixels in the direction of rows (horizontalscanning) may be arranged on both lateral sides of the pixel array 10 ina manner that allows the two horizontal scanning units operateseparately.

In a typical column parallel arrangement, each CDS unit and A/Dconverter are arranged on a per vertical column basis in a column areain an output portion of the image pickup device, and signals aresuccessively outputted to an output side of the device. The presentinvention is not limited to the column type. For example, a single CDSunit and a single A/D converter are assigned to a plurality of adjacentvertical signal lines 19 (for example, a pair of adjacent verticalsignal lines 19), or N vertical signal lines 19 (N is an positiveinteger number).

In any types other than the column type, a single CDS unit and a singleA/D converter are shared by a plurality of vertical signal lines 19, anda switch circuit is arranged to selectively feed one of pixel signals ofa plurality of columns from the pixel array 10 to the corresponding A/Dconverter. Depending on a subsequent process, a memory storing an outputsignal may be required.

A single CDS unit and a single A/D converter are shared by a pluralityof vertical signal lines 19 in this way. Signal processing is performedeach pixel signal after the pixel signal is read on a per pixel columnbasis. In comparison with the case in which signal processing isperformed in each unit pixel, simple structure of each unit pixel,multiple pixel design, miniaturization, and low cost design are easy toimplement.

Since pixel signals at one row are processed concurrently in parallel ina plurality of signal processors arranged in a column parallel fashion,signal processing is permitted to be operated at a speed slower than inthe arrangement in which a single CDS unit and a single A/D converteroutside an output circuit and a device. This is advantageous in terms ofpower consumption, bandwidth performance, and noise. Given the samepower consumption and the bandwidth, the entire sensor is permitted tooperate faster.

The column type device, which is permitted to operate a slow speed, isadvantageous not only in power consumption, bandwidth performance, andnoise, but also in design free from a switch circuit. Unless otherwisenoted in the following description, the solid-state image pickup deviceis the column type.

As shown in FIG. 1, the solid-state image pickup device 1 of the firstembodiment includes pixel array 10 having a plurality of unit pixels 3arranged in a matrix, a driver 7 arranged outside the pixel array 10, acolumn processor 26, a reference signal generator 27 for feeding areference voltage for A/D conversion to the column processor 26, and anoutput circuit 28.

As necessary, an automatic gain control (AGC) circuit having a signalamplification function may be arranged in front of or at the back of thecolumn processor 26 in the same semiconductor area. The AGC circuit, ifarranged in front of the column processor 26, performs an analogamplification. The AGC circuit, if arranged at the back of the columnprocessor 26, performs a digital amplification. If n-bit digital data ismerely amplified, gradation of the data can be destroyed. It istherefore preferred to perform the A/D conversion after the analogamplification.

The driver 7 has a control circuit function for successively reading asignal from the pixel array 10. The driver 7 includes a horizontal(column) scanning circuit 12 for controlling column addresses and columnscanning, a vertical (row) scanning circuit 14 for controlling rowaddresses and row scanning, and a communication and timing controller 20for generating internal clocks.

A clock converter 23 enclosed in a broken-lined box may be arranged asshown near the communication and timing controller 20 in FIG. 1. Theclock converter 23 as an example of a high-speed clock generatorgenerates a pulse having a higher clock frequency than a clock frequencyof a signal input thereto. The communication and timing controller 20generates the internal clocks in response to an input clock (masterclock) CLK0 inputted via a terminal 5 a or a high-speed clock generatedby the clock converter 23.

The use of the high-speed clock generated by the clock converter 23allows the A/D conversion process to be executed at a fast speed. Withthe high-speed clock, an extraction process and a compression process,each requiring high-speed calculations, can be performed. Parallel dataoutputted from the column processor 26 can be converted into serial datato be outputted outside the device as video data D1. In this way, thevideo data is outputted at terminals less than terminals equal to thenumber of bits of the A/D converted digital data.

The clock converter 23 includes a frequency multiplier circuit forgenerating a pulse having a clock frequency higher than a clockfrequency of the input clock. Upon receiving a low-speed clock CLK2 fromthe communication and timing controller 20, the clock converter 23generates a clock having a frequency at least twice the low-speed CLK2.The frequency multiplier circuit of the timing controller 21 is a k1multiplier circuit, where k1 represents the frequency of the low-speedclock CLK2. The k1 multiplier circuit can be any of known circuits.

Although columns and rows of the pixels are partially shown in FIG. 1,unit pixels 3 of tens to several thousands are arranged in practice. Theunit pixel 3 typically includes a photodiode, such as a light receivingelement (charge generator), and a pixel amplifier including anamplifying semiconductor element (such as a transistor).

The pixel amplifier may have a floating diffusion amplifier structure.The charge generator typically includes commercially available fourtransistors as a CMOS sensor, namely, a read and selection transistorfunctioning as a charge reader (a transfer gate section/a read section),a reset transistor functioning as a reset gate, a vertical selectiontransistor, and an amplifying source-follower transistor functioning asa detector element for detecting a voltage change in floating diffusion.

The pixel amplifier may be composed of three transistors as disclosed inJapanese Patent No. 2708455. The pixel amplifier includes an amplifyingtransistor, connected to drain (DRN) line, for amplifying a signalvoltage corresponding to a signal charge generated in a chargegenerator, a reset transistor for resetting the charge generator, a readand selection transistor (transfer gate) being scanned by a verticalshift transistor via transfer (TRF) line.

The driver 7 further includes the horizontal scanning circuit 12, thevertical scanning circuit 14, and the communication and timingcontroller 20. The horizontal scanning circuit 12 has a read andscanning function to read a count from the column processor 26. Alongwith the pixel array 10, the elements in the driver 7 are integrallyproduced on a semiconductor area such as a monocrystal silicon substrateinto a solid-state image pickup device using the same technique as thesemiconductor integrated circuit manufacturing process.

The unit pixel 3 is connected to the vertical scanning circuit 14 via arow control line 15 for selecting a row, and via vertical signal line 19to the column processor 26 having a column A/D converter 25 arranged ona per vertical column basis. The row control line 15 represents alllines routed from the vertical scanning circuit 14 to the pixels.

Each of the horizontal scanning circuit 12 and the vertical scanningcircuit 14, including a decoder, starts a shift operation (scanning) inresponse to control signals CN1 an CN2 provided by the communication andtiming controller 20. The row control line 15 conducts a variety ofpulse signals for driving the unit pixel 3 (such as a reset pulse RST, atransfer pulse TRF, a DRN control pulse DRN, etc.).

The communication and timing controller 20 includes a timing generatorTG (as an example of a read address controller) for supplying clocksrequired for various operations and pulse signals having a predeterminedtiming and a communication interface for receiving the master clock CLK0via the terminal 5 a, and data DATA for instructing an operation modevia the terminal 5 a, and for outputting data containing informationrelating to the solid-state image pickup device 1. The timing generatorTG and the communication interface are not shown in FIG. 1.

A horizontal address signal is outputted to a horizontal decoder 12 a,and a vertical address signal is outputted to a vertical decoder 14 a.In response, the horizontal decoder 12 a and the vertical decoder 14 aselect respective row and column.

The unit pixels 3 are arranged in a two-dimensional matrix, and verticalscanning for reading is performed by accessing the unit pixels 3 on aper row basis (in a column parallel fashion) to capture the analog pixelsignal, generated by a pixel signal generator 5 and outputted via thevertical signal lines 19 in the direction of columns, and thenhorizontal scanning for reading is performed by accessing the unitpixels 3 in the directions of columns to read pixel signals (digitalpixel data). The pixel signal and the pixel data are fast read. Thereading operation is not limited to scanning. Random accessing forreading information of only a targeted unit pixel 3 can be performed bydirectly addressing the targeted unit pixel 3.

The communication and timing controller 20 of the first embodimentsupplies to internal elements of the device including the horizontalscanning circuit 12, the vertical scanning circuit 14, and the columnprocessor 26, a clock CLK1 having the same frequency as the master clockCLK0 supplied via the terminal 5 a, and a low-speed clock that isobtained by frequency dividing the clock CLK1 by 2. Hereinafter, thedivide-by-2 clock and other clocks having lower frequencies arecollectively referred to as a low-speed clock CLK2.

The vertical scanning circuit 14 selects a row of the pixel array 10,and supplies that row with required pulses. The vertical scanningcircuit 14 includes the vertical decoder 14 a and a vertical drivingcircuit 14 b. The vertical decoder 14 a defines a read row in thevertical direction (selects a row of the pixel array 10). The verticaldriving circuit 14 b supplies pulses to the row control line 15, therebydriving the unit pixels 3 on the read address defined by the verticaldecoder 14 a. The vertical decoder 14 a selects an electronic shutter inaddition to the reading of the signal.

The horizontal scanning circuit 12 successively selects the column A/Dconverters 25 of the column processor 26 in synchronization with thelow-speed clock CLK2, transferring the signal from the column A/Dconverter 25 to the horizontal signal line 18. The horizontal scanningcircuit 12 includes a horizontal decoder 12 a for defining a read columnin the horizontal direction (selecting the column A/D converter 25within the column A/D converter 25), and a horizontal driving circuit 12b for transferring the signal of the column processor 26 to thehorizontal signal line 18. The horizontal signal line 18 includes nlines if the number of bits handled by the column A/D converter 25 is nbits, for example, 10 lines if the number of bits is 10.

In the solid-state image pickup device 1 thus constructed, the pixelsignal outputted from the unit pixel 3 is supplied to the column A/Dconverter 25 in the column processor 26 via the vertical signal line 19on a per vertical column basis.

The column A/D converter 25 in the column processor 26 processes thepixel signal of one column. For example, the column A/D converter 25includes an A/D converting circuit for converting an analog signal into10-bit digital data using the low-speed clock CLK2.

The A/D converting circuit to be discussed later in detail startscounting the clock signal in response to a reference voltage RAMPsupplied to the voltage comparator 252 thereof, and compares thereference voltage RAMP with the analog pixel signal inputted via thevertical signal line 19. The A/D converting circuit performs A/Dconversion by continuously counting until the pulse signal is obtained.

By introducing an appropriate circuit, a difference between a signallevel (noise level) immediately subsequent to pixel resetting withrespect to a pixel signal in a voltage mode inputted via the verticalsignal line 19 and a true signal level Vsig responsive to an amount oflight is determined. In this way, noise signal component such as a fixedpattern noise (FPN) and reset noise is thus removed.

The pixel data digitized by the column A/D converter 25 is transferredto the horizontal signal line 18 via a horizontal selection switch (notshown) driven by a horizontal selection signal, and then inputted to theoutput circuit 28. The number of bits of 10 is only an example. Thenumber of bits may be any number above 10 (14 bits, for example).

Pixel signals are thus successively outputted on a per row basis fromthe pixel array 10 having a matrix of light receiving elements as acharge generator. A frame image corresponding to the pixel array 10having the matrix of the photosensitive elements is thus obtained as aset of pixel signals of the whole pixel array 10.

The reference signal generator 27 includes a digital-to-analog (D/A)converter 27 a. In synchronization with the count clock CK0, thereference signal generator 27 generates a ramp signal from the initialvalue represented by control data CN 4 from the communication and timingcontroller 20. The reference signal generator 27 then supplies thegenerated ramp signal as a reference voltage (A/D reference signal) forA/D conversion to each column A/D converter 25 in the column processor26. A noise suppression filter (not shown) may also be arranged.

The ramp signal is varied faster by using the high-speed clock from theclock converter 23, for example, the frequency multiplied clockgenerated by the frequency multiplier in the clock converter 23, than byusing the count clock CLK0.

The control data CN4 supplied from the communication and timingcontroller 20 to the D/A converter 27 a in the reference signalgenerator 27 contains information for equalizing a rate of change ofdigital data with respect to time, namely, for equalizing the slope ofthe ramp voltage from comparison process to comparison process. Morespecifically, the digital data is varied one count per unit time.

The column A/D converter 25 having an n-bit A/D conversion functionincludes a voltage comparator 252 and a counter 254. The voltagecomparator 252 compares the reference voltage RAMP generated by the D/Aconverter 27 a in the reference signal generator 27 with the analogpixel signal supplied from the unit pixels 3 on a per row control line15 (H0, H1, . . . ) via the vertical signal lines 19 (V0, V1, . . . ).The counter 254 counts time until the voltage comparator 252 completesthe comparison operation, and stores the resulting count.

The communication and timing controller 20 switches the count mode ofthe counter 254 depending on whether the voltage comparator 252 performsthe comparison operation on either the reset component ΔV or the signalcomponent Vsig of the pixel signal. A control signal CN5 is suppliedfrom the communication and timing controller 20 to the counter 254 inthe column A/D converter 25 to give the counter 254 an instruction ofwhich count mode to perform, the down-count mode or the up-count mode.

The reference voltage RAMP generated by the reference signal generator27 is fed to input terminals RAMP of the voltage comparators 252. Theother input terminals of the voltage comparators 252, connected torespective vertical signal lines 19 of the vertical columns, receiveindividual pixel signal voltages from the pixel array 10. The output ofthe voltage comparator 252 is fed to the counter 254.

The communication and timing controller 20 supplies a count clock CK0commonly to clock terminals CK of the counters 254.

The counter 254, although the structure thereof not shown here, isconstructed by modifying the wiring of the data memory 255 having thelatches as shown in FIG. 6 into a wiring for a synchronization counter.Internal counting is performed in response to the inputting of a singlecount clock CK0. As the ramp signal, the count clock CK0 is generatedbased on the high-speed clock (frequency-multiplied clock) from theclock converter 23, and thus faster than the master clock CLK0.

A combination of n latches forms the counter 254. As a result, theresulting circuit has half the size of the circuit of the data memory255 having the two lines of n latches as shown in FIG. 6. Since thecounter 24 is eliminated, the circuit becomes much more compact than thearrangement of FIG. 6.

The counter 254 of the first embodiment is an up-down counter (U/D CNT)that can be switched between the down-count mode and the up-count mode.The counter 254 of the first embodiment also is a synchronizationcounter type that outputs the count in synchronization with the countclock CK0.

In the synchronization counter, operation of all flipflops is controlledby the count clock CK0. If a higher frequency operation is required, anasynchronous counter is preferable as the counter 254. The asynchronouscounter is appropriate for use in high-frequency range because theoperational limiting frequency thereof is determined by a limitingfrequency of only an initial-stage flipflop.

A control pulse is fed from the horizontal scanning circuit 12 to thecounter 254 via a control line 12 c. The counter 254, having a latchfunction for latching the count thereof, stores the count until aninstruction in the form of the control pulse is supplied via the controlline 12 c.

As previously discussed, the column A/D converters 25, each arranged foreach vertical signal line 19 (V0, V1, . . . ), form the column processor26 functioning as an A/D converter block.

The output of the column A/D converter 25 is connected to the horizontalsignal line 18. The horizontal signal line 18 includes signal lines of nbits equal to the bit width of the column A/D converter 25. Thehorizontal signal line 18 is thus connected to the output circuit 28 vian sense circuits (not shown) corresponding to the horizontal signal line18.

The column A/D converter 25 performs a count operation during a pixelsignal read period corresponding to a horizontal blanking period,thereby outputting the counting result at a predetermined timing. Thevoltage comparator 252 compares the ramp voltage from the referencesignal generator 27 with the pixel signal voltage input via the verticalsignal line 19. If both voltages become equal in level, the voltagecomparator 252 inverts the comparator output thereof from a high levelto a low level in this embodiment.

The counter 254 has already started one of the down-counting operationand the up-counting operation in synchronization with the ramp voltageissued from the reference signal generator 27. Upon receiving theinformation of the inversion of the comparator output, the counter 254stops the counting operation thereof, and latches the count at themoment as pixel data. The A/D conversion thus ends.

The counter 254 outputs the stored pixel data via the output terminal 5c outside the column processor 26 and the chip of the pixel array 10 inaccordance with a shift operation caused by a horizontal selectionsignal CH(i) that is inputted from the horizontal scanning circuit 12via the control line 12 c at a predetermined timing.

Other signal processing circuits (not shown) unrelated to the firstembodiment of the present invention may be contained in the solid-stateimage pickup device 1.

FIG. 2 is a timing diagram illustrating operation of the column A/Dconverter 25 in the solid-state image pickup device 1 of FIG. 1 inaccordance with the first embodiment of the present invention.

The analog pixel signal detected by each unit pixel 3 in the pixel array10 is A/D converted as below. A point where the reference voltage RAMPthat gradually falls at a predetermined slope matches the voltage of oneof a standard component and a signal component of the pixel signal fromthe unit pixel 3 is detected. The count clock from the start of thegeneration of the reference voltage RAMP for use in the A/D conversionto the point where the reference signal matches the signal correspondingto one of the standard component and the signal component in the pixelsignal is counted. The count responsive to the magnitude of the standardcomponent or the signal component is thus obtained.

In the pixel signal output from the vertical signal line 19, the signalcomponent Vsig appears subsequent to the reset component ΔV containingnoise of the pixel signal as the standard component. When a firstprocess is performed on the standard component (reset component ΔV), asecond process is performed on the sum of the standard component (resetcomponent ΔV) and the signal component Vsig.

To perform first reading, the communication and timing controller 20resets the count of the counter 254 to “0” while setting the counter 254to the down-count mode. After the first reading from the any row Hx ofthe unit pixels 3 to the vertical signal line 19 (V0, V1, . . . )becomes stabilized, the communication and timing controller 20 suppliesthe control data CN4 for generating the reference voltage RAMP to thereference signal generator 27.

In response, the reference signal generator 27 feeds the ramp waveformvaried in time in a ramp shape to the RAMP terminals of the voltagecomparators 252 as a comparison voltage. The voltage comparator 252compares the comparison voltage having the ramp waveform with the pixelsignal voltage supplied from the pixel array 10 via any vertical signalline 19 (Vx).

When the reference voltage RAMP is inputted to the input terminal RAMPof each of the voltage comparator 252, the counters 254 arranged at eachrow start counting the comparison time. In synchronization with the rampshaped voltage issued from the reference signal generator 27 (time t10),the communication and timing controller 20 inputs the count clock CK0 tothe clock terminal of the counter 254. In a first counting operation,the counter 254 starts down-counting with an initial value “0”, namely,performing the counting operation in a negative direction.

The voltage comparator 252 compares the reference voltage RAMP from thereference signal generator 27 with the pixel signal voltage Vx input viathe vertical signal line 19, and inverts the comparator output thereoffrom a high level to a low level (time t12) when the two voltages becomeequal. More specifically, the voltage comparator 252 compares thevoltage signal responsive to the reset component Vrst with the referencevoltage RAMP, thereby generating an active low pulse having a length intime axis corresponding to the magnitude of the reset component Vrst.The active low pulse is then supplied to the counter 254.

In response, the counter 254 stops the counting operation at about thesame time as the inversion of the comparator output. The count at themoment is latched. The A/D conversion is thus complete (time t12). Thewidth of the active low pulse signal having the length in time axisobtained as a result of the comparison operation by the voltagecomparator 252 is counted with respect to the clock count CK0. The countresponsive to the reset component Vrst is thus obtained.

At the moment a predetermined down-count period has elapsed (time t14),the communication and timing controller 20 stops the supply of thecontrol data to the voltage comparator 252 and the supply of the countclock CK0 to the counter 254. In this way, the voltage comparator 252stops the comparison operation.

During the first reading, the counting operation is performed bydetecting the reset level Vrst of the pixel signal voltage Vx with thevoltage comparator 252. As a result, the reset component ΔV of the unitpixel 3 is read.

The reset component ΔV contains noise varying from the unit pixel 3 tothe unit pixel 3 as an offset. The variations in the unit pixel 3 aretypically small, and the reset level Vrst is typically common to allpixels. The output value of the reset component ΔV in the pixel signalvoltage Vx of the any vertical signal line 19 is largely known.

During the first reading of the reset component ΔV, a down-count period(comparison period from time t10 to time t14) can be shortened byadjusting the reference voltage RAMP. In this embodiment, the maximumlength of the comparison operation for the reset component ΔV is set tobe a 7-bit count period (128 clocks).

During a second reading, the signal component Vsig responsive to anamount of input light to each unit pixel 3 is read in addition to thereset component ΔV, and the same process as the first reading isperformed. The communication and timing controller 20 sets the counter254 to the up-count mode. When the second reading of the unit pixels 3at any row Hx to the vertical signal line 19 (V0, V1, . . . ) isstabilized, the communication and timing controller 20 supplies thecontrol data CN4 for generating the reference voltage RAMP to thereference signal generator 27.

In response, the reference signal generator 27 feeds the ramp waveformvaried in time in a ramp shape to the RAMP terminals of the voltagecomparators 252 as a comparison voltage. The voltage comparator 252compares the comparison voltage having the ramp waveform with the pixelsignal voltage supplied from the pixel array 10 via any vertical signalline 19 (Vx).

When the reference voltage RAMP is inputted to the input terminal RAMPof each of the voltage comparator 252, the counters 254 arranged at eachrow start counting the comparison time. In synchronization with the rampshaped voltage issued from the reference signal generator 27 (time t20),the communication and timing controller 20 inputs the count clock CK0 tothe clock terminal of the counter 254. In a second counting operation,the counter 254 starts up-counting with the count corresponding to thereset component ΔV of the unit pixel 3 acquired during the firstreading, namely, performing the counting operation in a positivedirection.

The voltage comparator 252 compares the reference voltage RAMP from thereference signal generator 27 with the pixel signal voltage Vx inputtedvia the vertical signal line 19, and inverts the comparator outputthereof from a high level to a low level (time t22) when the twovoltages become equal. More specifically, the voltage comparator 252compares the voltage signal responsive to the signal component Vsig withthe reference voltage RAMP, thereby generating an active low pulsehaving a length in time axis corresponding to the magnitude of thesignal component Vsig. The active low pulse is then supplied to thecounter 254.

In response, the counter 254 stops the counting operation at about thesame time as the inversion of the comparator output. The count at themoment is latched. The A/D conversion is thus complete (time t22). Thewidth of the active low pulse signal having the length in time axisobtained as a result of the comparison operation by the voltagecomparator 252 is counted with respect to the clock count CK0. The countresponsive to the signal component Vsig is thus obtained.

At the moment a predetermined up-count period has elapsed (time t24),the communication and timing controller 20 stops the supply of thecontrol data to the voltage comparator 252 and the supply of the countclock CK0 to the counter 254. In this way, the voltage comparator 252stops the comparison operation.

During the second reading, the counting operation is performed bydetecting the signal component Vsig in the pixel signal voltage Vx withthe voltage comparator 252. As a result, the signal component Vsig ofthe unit pixel 3 is read.

In accordance with the first embodiment of the present invention, thecounter 254 performs the down-counting operation in the first reading,and the up-counting operation in the second reading. The counter 254thus automatically performs a subtraction process expressed by equation(1), and then stores the count as the subtraction result:(Count in the second comparison)−(count in the first comparison)  (1)

Equation (1) is also expressed as equation (2), and the count stored inthe counter 254 as a result is the signal component Vsig as follows:(Count in the second comparison)−(count in the first comparison)=(signalcomponent Vsig+reset component ΔV+offset component of the column A/Dconverter 25)−(reset component ΔV L+offset component of the column A/Dconverter 25)=(signal component Vsig)  (2)

The two reading operations and the subtraction process of the counter254, such as the down-counting operation in the first reading and theup-counting operation in the second reading, removes the reset componentΔV containing variations from the unit pixel 3 to the unit pixel 3 andthe offset component of individual column A/D converters 25. Only thesignal component Vsig responsive to the amount of input light iscaptured on a per unit pixel basis using a simple arrangement. Resetnoise is also removed.

The column A/D converter 25 of the first embodiment functions not onlyas a D/A converter for converting the analog pixel signal into thedigital data, but also as the CDS unit.

The count obtained in equation (2) shows that the pixel data has apositive value, and no complementary operation is required. The firstembodiment of the present invention thus achieves high compatibilitywith an existing system.

In the second reading, the signal component Vsig responsive to theamount of incident light is read. To measure the level of the amount ofincident light in a wide range, an up-count period (comparison period oft20 to t24) needs to be widened so that the ramp voltage to be suppliedto the voltage comparator 252 varies greatly.

In accordance with the first embodiment, the maximum length of thecomparison operation for the signal component Vsig is set to be a 10-bitcount period (1024 clocks). In other words, the maximum length of thecomparison process of the reset component ΔV is set to be shorter thanthe comparison process of the signal component Vsig rather than beingset to be equal to the comparison process of the signal component Vsig.As a result, the total length of the two A/D conversion operations isshortened.

Although the number of bits is different from the first comparisonoperation to the second comparison operation, the slopes (rates ofchange) of the reference voltages RAMP of the first and secondcomparison operations are set to be equal to each other. Thisequalization is performed by supplying digital data from thecommunication and timing controller 20 to the reference signal generator27 so that the reference signal generator 27 generates the referencevoltage RAMP based on the control data. Since the generation of thereference voltage RAMP is digitally controlled, the slopes of the firstand second reference voltages RAMP are easily equalized. The accuraciesof A/D conversions are set to be equal to each other, and thesubtraction results of the up-down counter resulting from equation (1)are correctly obtained.

At a predetermined timing (time t28) after the completion of the secondcounting operation, the communication and timing controller 20 instructsthe horizontal scanning circuit 12 to read the pixel data. In response,the horizontal scanning circuit 12 successively shifts the horizontalselection signal CH(i) to be supplied to the counter 254 via the controlline 12 c.

The count represented in equation (2) and stored in the counter 254,namely, n-bit digital data as the pixel data, is successively outputtedfrom the column processor 26 or the chip housing the pixel array 10 viathe output terminal 5 c. The same process is repeated, thereby producingthe video data D1 representing a two-dimensional image.

The solid-state image pickup device of the first embodiment uses theup-down counter with the count mode thereof switched, thereby performingthe two counting operations. In the arrangement of the matrix of unitpixels 3, the column A/D converter 25 is constructed of a columnparallel A/D circuit on a per vertical column basis.

The subtraction operation between the standard component (resetcomponent) and the signal component is directly obtained on a per columnbasis as the result of the second counting. The memory function ofstoring the count results of the standard component and the signalcomponent are performed by the latches in the counter. This arrangementeliminates the need for arranging a dedicated memory for storing the A/Dconverted data.

No dedicated subtracter for determining a difference between thestandard component and the signal component is required. The scale andarea of the circuit are reduced in comparison with the conventional art.Increases in noise, current, and power consumption are avoided.

Since the column processor 26 is composed of the voltage comparator 252and the counter 254, the counting operation is controlled by the countclock line and the control line for operating the counter 254 regardlessof the number of bits. Signal lines for transferring a count from acounter to a memory, required in the conventional art, are eliminated.Increases in noise and power consumption are thus avoided.

More specifically, the solid-state image pickup device 1 includes thecolumn A/D converter 25 composed of the voltage comparator 252 and thecounter 254. The counter 254 is switched between the down-count mode andthe up-count mode to determine the difference digital data between thestandard component (the reset component in the first embodiment) and thesignal component. This arrangement prevents increases in circuit scale,circuit area, and power consumption, the number of interface wiringlines with another apparatus, and noise and power consumption caused bythe interfacing wiring lines.

FIG. 3 illustrates a CMOS solid-state image pickup device in accordancewith a second embodiment of the present invention. The solid-state imagepickup device 1 of the second embodiment includes a modification in thecolumn A/D converter 25 in the solid-state image pickup device 1 of thefirst embodiment of the present invention.

The column A/D converter 25 of the second embodiment includes a n-bitdata memory 256, arranged in a subsequent stage to the counter 254, forstoring the count result stored in the counter 254, and a switch 258arranged between the counter 254 and the data memory 256.

The switches 258 are commonly supplied with a memory transfer commandpulse CN8 as a control pulse at a predetermined timing by thecommunication and timing controller 20. Upon receiving the memorytransfer command pulse CN8, the switch 258 transfers the count of thecorresponding counter 254 to the data memory 256. The data memory 256stores the incoming count.

The mechanism of storing the count of the counter 254 to the data memory256 at a predetermined timing is not limited to the switch 258. Forexample, the counter 254 is directly connected to the data memory 256and an output enable terminal of the counter 254 is controlled by thememory transfer command pulse CN8. Alternatively, the memory transfercommand pulse CN8 can be used as a latch clock to determine a datacapturing timing of the data memory 256.

The data memory 256 receives a control pulse from the horizontalscanning circuit 12 via the control line 12 c. The data memory 256stores the count, acquired from the counter 254, until the instructionin the form of the control pulse coming in via the control line 12 c.

The horizontal scanning circuit 12, having a read and scan function,reads the count stored in each data memory 256 in parallel with theprocesses of the voltage comparator 252 and the counter 254 in thecolumn processor 26.

In accordance with the second embodiment, the count result stored in thecounter 254 is transferred to the data memory 256. The countingoperation, i.e., the A/D conversion of the counter 254 and the readingoperation of the counter 254 to the horizontal signal line 18 can beindependently controlled. A pipeline operation in which the A/Dconversion and the reading of the signal are performed in parallel iscarried out.

FIG. 4 is a timing diagram illustrating operation of the column A/Dconverter 25 in the solid-state image pickup device 1 of the secondembodiment. The A/D conversion process of the column A/D converter 25remains unchanged from that of the first embodiment, and the detaileddescription thereof is omitted herein.

In accordance with the second embodiment, the data memory 256 is addedto the arrangement of the first embodiment. The basic operation of thesecond embodiment is identical to that of the first embodiment. Prior tothe operation of the counter 254 (at time t30), the count at a prior rowHx-1 is transferred to the data memory 256 in response to the memorytransfer command pulse CN8 from the communication and timing controller20.

In accordance with the first embodiment, the pixel data cannot betransferred from the column processor 26 only after the completion ofthe A/D conversion. In accordance with the second embodiment, nolimitation is imposed on the reading process because the results of thepreceding subtraction operation are already transferred to the datamemory 256 prior to the first reading process (A/D conversion).

In this arrangement, the signal output operation from the data memory256 via the horizontal signal line 18 and the output circuit 28 and thereading operation of the current Hx and the counting operation of thecounter 254 are performed in parallel. Efficient signal outputting isthus performed.

The embodiments of the present invention have been discussed. Thepresent invention is not limited to the previously discussedembodiments. A variety of changes and modifications are possible withoutdeparting from the scope of the present invention. Such changes andmodifications fall within the scope of the present invention.

The previously described embodiments are not intended to limit the scopeof the present invention. All of combinations of elements described inconnection with the above-referenced embodiments are not always requiredto constitute the present invention. Only part of a combination of theelements in the above-described embodiments can be sufficient toconstitute the present invention.

In the above-referenced embodiments, the column A/D converter 25composed of the voltage comparator 252 and the counter 254 is arrangedin each vertical column to obtain the digital data. Alternatively, asingle column A/D converter 25 may be arranged for a plurality ofvertical columns.

In the above-referenced embodiments, the A/D converter 25 is arranged inthe column area on the output side of the pixel array 10. Alternatively,a single column A/D converter 25 is arranged in another area. Forexample, the analog pixel signal is outputted to the horizontal signalline 18, and downstream of the horizontal signal line 18, the analogsignal is converted to digital signal to be transferred to the outputcircuit 28.

In this case, as well, the reference voltage for A/D conversion iscompared with the signal containing the standard component and thesignal component, the counting operation is performed in one of thedown-count mode and the up-count mode in parallel with conversionoperation, and the count mode is switched depending on which of thestandard component and the signal component is being subjected to thecomparison operation at the end of the comparison operation. The digitaldata representing the difference between the standard component and thesignal component is thus obtained as a result of the two modes ofdown-count and the up-count.

The function of a memory for storing the counts for the standardcomponent and the signal component is performed by the latch arranged inthe counter. No dedicated memory for storing the A/D converted data isrequired. A single A/D converter is sufficient for all vertical columns.Although high-speed conversion performance is required of the converter,the circuit scale is reduced.

In the above-referenced embodiments, the second counting operationstarts with the count stored in the first counting operation. If asynchronous up-down counter outputting the count in synchronization withthe count clock CK0, no particular operation is required at the modeswitching.

If an asynchronous up-down counter, having a high-speed advantage thatan operating limiting frequency is determined only by a limitingfrequency of a first flipflop, is used, the count is destroyed when thecount mode is switched. A normal counting operation in succession to thecount at the moment of the count mode switching cannot be performed. Forthis reason, an adjustment unit enabling the second counting operationto start with the count stored in the first counting operation ispreferred. The adjustment unit is not discussed further herein.

In the above-referenced embodiments, the signal component Vsig appearsafter the reset component ΔV (standard component) in the pixel signal ofthe same pixel in time sequence. In the first process, the comparisonoperation and the down-counting operation are performed on the resetcomponent ΔV (standard component) so that the subsequent unit performs apositive signal (the higher the signal level, the large the positivevalue thereof). In the second process, the comparison operation and theup-counting operation are performed on the signal component Vsig.Regardless of the sequential order of appearance of the standardcomponent and the signal component, the combination and process order ofthe component to be processed and the count mode are optional. Dependingon the process order, the digital data obtained in the second processcan be negative. In such a case, a correction operation may beperformed.

The pixel array 10 may be required to read the reset component ΔVsubsequent to the signal component Vsig, and the subsequent unit may berequired to process a positive signal. In such a case, to achieveefficient processing, the comparison operation and the down-countoperation are performed on the signal component Vsig in the firstprocess, and the comparison operation and the up-count operation areperformed on the reset component ΔV (standard component) in the secondprocess.

In accordance with the above-referenced embodiments, the up-down counteris commonly used with the count mode switched between the up-count modeand the down-count mode. It is sufficient if a combination of thedown-count mode and the up-count mode is performed on the standardcomponent and the signal component. The present invention is not limitedto the use of the mode-switchable up-down counter.

For example, a counter may be composed of a down-counting circuit forperforming the down-counting operation subsequent to the comparisonoperation on one of the standard component and the standard component,and an up-counting circuit for performing the up-counting operationsubsequent to the comparison operation to the other of the standardcomponent and the signal component.

In such a case, the counting circuit for performing the second countingoperation is any of the known art that enables any initial value to beloaded. If the up-counting operation is performed subsequent to thedown-counting operation, the down-counter is operated in the firstprocess, and the up-counter is operated in the second process as shownin FIG. 5A.

Prior to the start of the up-counting operation subsequent to the countmode switching in response to the control signal CN5 for count modeswitching, a load terminal LDu of the up-counter is supplied with aswitch control signal CNload for setting an initial value. In this way,the down-count acquired in the down-counting operation is set in theup-counter.

If the down-counting operation is performed subsequent to theup-counting operation, the up-counter is operated in the first process,and the down-counter is operated in the second process as shown in FIG.5B.

Prior to the start of the down-counting operation subsequent to thecount mode switching in response to the control signal CN5 for countmode switching, a load terminal LDd of the down-counter is supplied witha switch control signal CNload for setting an initial value. In thisway, the up-count acquired in the up-counting operation is set in thedown-counter.

In any of the arrangements of FIGS. 5A and 5B, the output of the secondcounter is directly obtained as a result of the subtraction operationbetween the standard component and the signal component. No particularsubtracter for determining a difference between the standard componentand the signal component is required. The arrangement eliminates theneed for the data transfer to the subtracter, which is required in thenon-patent document 1. Increases in noise, current, and powerconsumption are avoided.

In the counter composed of a down-counting circuit and an up-countingcircuit, the second counting operation may start with zero rather thanthe count acquired in the first counting operation. In this case, asshown in FIG. 5C, an adder for adding an output Qup of the up-counteringcircuit and an output Qdown of the down-counting circuit is required.Since the adder is arranged in each A/D converter, the length of wiringlines is short. Increases in noise, current, and power consumption areavoided.

In each of the arrangements of FIGS. 5A-5C, the communication and timingcontroller 20 issues an operation command to the down-counting circuitand the up-counting circuit as in the preceding embodiments. Both thedown-counting circuit and the up-counting circuit are operated inresponse to the count clock CK0.

In accordance with the above-referenced embodiments, the unit element isconstructed of an NMOS. Alternatively, the unit element may beconstructed of PMOS. In such a case, the same advantages are alsoprovided with the voltage positive and negative relationship inverted.

In accordance with the above-referenced embodiments, the solid-stateimage pickup device reading a signal from an individual unit pixelthrough address control is the CMOS sensor for generating a signalcharge by receiving light. The signal charge is generated not only bylight but also by another type of general electromagnetic waves such asrays including infrared light, ultraviolet light, and X-rays. Thepresent invention is applicable to a semiconductor device including agreat deal of unit elements, each unit element outputting an analogsignal in response to the intensity of the electromagnetic wave.

In accordance with the above-referenced embodiments, the solid-stateimage pickup device includes the A/D converter including the comparatorand the counter. The comparator compares the reference voltage for A/Dconversion with each of the signals corresponding to the standardcomponent and the signal component. The counter performs the countingoperation in one of the down-count mode and the up-count mode inparallel with the comparison operation, and stores the count at themoment the comparison operation is complete. The A/D converter is notlimited to the solid-state image pickup device. The A/D converter isapplicable to any electronic apparatus that performs A/D converts adifference signal between two signal components.

The A/D converter is applicable not only to the solid-state image pickupdevice and the electronic apparatus, but also to an integrated circuitand an A/D converting module, which is typically supplied as anindividual device.

The A/D converter includes the comparator and the counter.Alternatively, the integrated circuit (IC) may further include on thesame semiconductor substrate thereof the reference signal generator forgenerating the reference signal for A/D conversion and supplying thereference signal to the comparator, and the controller for switching thecount mode of the counter depending on whether the comparator hasperformed the comparison operation on the standard component and thesignal component. Such an IC and individual chips may be constructedinto a module.

The use of the modular construction permits functional units required tocontrol the comparator and the counter to be collectively integrated.The handling and management of the modules are thus easy. Since theelements required for the A/D conversion are combined in an IC or amodule in a unitary body, the manufacture of the solid-state imagepickup device and other electronic apparatuses is easy.

1. A method for analog-to-digital converting, into digital data, adifference signal component, between a standard component and a signalcomponent, of an analog signal to be processed containing the standardcomponent and the signal component, the method comprising a first stepand a second step performed subsequent to the first step, wherein thefirst step comprises comparing a signal corresponding to one of thestandard component and the signal component with a reference signal forgenerating the digital data, performing a counting operation in one of adown-count mode and an up-count mode in parallel with the signalcomparison operation, and then storing the resulting count at the momentthe comparison operation is completed, and wherein the second stepcomprises comparing the other of the standard component and the signalcomponent with the reference signal, performing a counting operation inthe other of the down-count mode and the up-count mode in parallel withthe signal comparison operation, and then storing the resulting count atthe moment the comparison operation is completed.
 2. The methodaccording to claim 1, wherein an up-down counter is commonly used toperform the counting operation in the up-count mode and the down-countmode with the operation mode thereof switched.
 3. The method accordingto claim 1, wherein the counting operation in the second step startswith the count stored in the first step.
 4. The method according toclaim 1, wherein the comparison operation and the counting operation inthe first step are performed on the standard component, and wherein thecomparison operation and the counting operation in the second step areperformed on the signal component.
 5. The method according to claim 1,wherein one of the first step and the second step is performed on thestandard component in the down-count mode, and wherein the other of thefirst step and the second step is performed on the signal component inthe up-count mode.
 6. The method according to claim 1, wherein themaximum period of the comparison operation for the standard component isshorter than the maximum period of the comparison operation for thesignal component.
 7. The method according to claim 1, wherein variationcharacteristics of the signal component are set to be equal from thefirst step to the second step.
 8. The method according to claim 1,wherein the count stored in the second step in connection with onesignal processed in a preceding cycle of the first step and the secondstep is stored in another data memory, and wherein when the first stepand the second step are performed on the other signal to be processed ina current cycle, the count is read from the other data memory inparallel.
 9. The method according to claim 1, wherein the signal to beprocessed contains an analog unit signal generated and output in thedirection of columns by a unit signal generator in a semiconductordevice for detecting a distribution of a physical quantity, wherein thesemiconductor device includes a matrix of unit elements, each unitelement including a charge generator for generating a chargecorresponding to an input electromagnetic wave, and the unit signalgenerator for generating the analog unit signal corresponding to thecharge generated by the charge generator.
 10. The method according toclaim 9, wherein the analog unit signal generated and output in thedirection of columns by the unit signal generator is captured on a perrow basis, and wherein the first step and the second step are performedfor each unit element on a per row basis.
 11. The method according toclaim 9, wherein the standard component comprises a reset componentcontaining noise of the unit signal.
 12. A converter foranalog-to-digital converting, into digital data, a difference signalcomponent, between a standard component and a signal component, of ananalog signal to be processed containing the standard component and thesignal component, the converter comprising: a comparator for comparing asignal corresponding to each of the standard component and the signalcomponent with a reference signal for generating the digital data; and acounter for performing a counting operation in one of a down-count modeand an up-count mode in parallel with the comparison operation performedby the comparator and for storing the resulting count at the moment thecomparison operation is completed.
 13. The converter according to claim12, further comprising a reference signal generator for generating thereference signal for generating the digital signal and supplying thereference signal to the comparator.
 14. The converter according to claim12, further comprising a controller for switching the count mode of thecounter depending on whether the comparator performs the comparisonoperation on the standard component or the signal component.
 15. Theconverter according to claim 12, wherein the counter is composed of acommon counting circuit that switchably performs the counting operationbetween an up-count mode and a down-count mode.
 16. The converteraccording to claim 14, therein the controller causes the counter tostart the counting operation in a second step with a count stored in afirst step.
 17. The converter according to claim 14, wherein thecontroller performs a control process to perform the comparisonoperation and the counting operation on the standard component in afirst step, and to perform the comparison operation and the countprocess on the signal component in a second step.
 18. The converteraccording to claim 14, wherein the controller switches the countingoperation of the counter so that the counter performs the countingoperation in the down-count mode if the comparator performs thecomparison operation on the standard component, and so that the counterperforms the counting operation in the up-count mode if the comparatorperforms the comparison operation on the signal component.
 19. Theconverter according to claim 13, wherein the reference signal generatorequalizes variation characteristics in the reference signal from thefirst step to the second step.
 20. The converter according to claim 12,further comprising: a data memory for storing the count stored in thecounter in connection with the signal processed in a first step; and ascanning unit for reading the count from the data memory in parallelwith the comparison operation of the comparator and the countingoperation of the counter, both performed on a signal to be processed ina second step.
 21. A semiconductor device, for detecting a distributionof a physical quantity, including a matrix of unit elements, each unitelement including a charge generator for generating a chargecorresponding to an input electromagnetic wave, and a unit signalgenerator for generating an analog unit signal containing a standardcomponent and a signal component, the semiconductor device comprising: acomparator for comparing a reference signal for generating digital dataof the signal component with each of signals corresponding to thestandard component and the signal component; and a counter forperforming a counting operation in one of a down-count mode and anup-count mode in parallel with the comparison operation of thecomparator, and storing a count at the moment at the moment thecomparator completes the comparison operation.
 22. The semiconductordevice according to claim 21, further comprising a reference signalgenerator for generating the reference signal for generating the digitalsignal and supplying the reference signal to the comparator.
 23. Thesemiconductor device according to claim 21, further comprising acontroller for switching the count mode of the counter depending onwhether the comparator performs the comparison operation on the standardcomponent or the signal component.
 24. The semiconductor deviceaccording to claim 21, further comprising a plurality ofanalog-to-digital converters arranged in a direction of columns and adirection of rows of unit elements, wherein each analog-to-digitalconverter comprises the comparator and the counter.
 25. Thesemiconductor device according to claim 21, wherein the comparatorcaptures, on a per row basis, the analog unit signal generated andoutput in the direction of columns by the unit signal generator; andwherein the comparator and the counter perform respective processes foreach unit element on a per row basis.
 26. The semiconductor deviceaccording to claim 21, wherein the comparator captures, as the standardcomponent, a reset component containing noise of the unit signal. 27.The semiconductor device according to claim 21, wherein the chargegenerator comprises a photoelectric conversion element that receiveslight as an electromagnetic wave and generates a charge responsive tothe received light.
 28. The semiconductor device according to claim 21,wherein the unit signal generator comprises a semiconductor element foramplification.
 29. An electronic apparatus comprising: a referencesignal generator for generating a reference signal for converting, intodigital data, a difference signal component, between a standardcomponent and a signal component, of an analog signal containing thestandard component and the signal component; a comparator for comparingthe reference signal generated by the reference signal with each ofsignals corresponding to the standard component and the signalcomponent; a counter for performing a counting operation in one of adown-count mode and an up-count mode in parallel with the comparisonoperation of the comparator, and storing a count at the moment thecomparator completes the comparison operation; and a controller forswitching the count mode of the counter between the up-count mode andthe down-count mode depending on whether the comparator performs thecomparison operation on the standard component or the signal component.